If you cut your arrows to measure 28.5" to the back of the point (one inch longer than your draw length):
Adjusting spine for your draw length, length of the arrow, and the Thunderstick's being cut 1/16" before center, my best guess is 50/55's. That is based on how my Thunderstick Mag shoots wood shafts. And if you cut them yourself, I would even start with the arrow at 29" to back of point, and shorten if needed.
Another option would be 55/60 spine, cut one inch longer than the 50/55's. This would allow for a stiffer spined shaft by shortening, although I find my Thunderstick to prefer a slightly softer spine.
Just my experience-I am not an expert.